Mask substrate, projection exposure apparatus equipped with the mask substrate, and a pattern formation method utilizing the projection exposure apparatus

ABSTRACT

In order to reduce mask pattern transfer error caused by expansion of the mask substrate during transferring the circuit pattern onto the photosensitive substrate, a mask substrate is loosely supported on a plurality of mounts on the mask stage so that the mask substrate can freely expand in response to changes in its temperature. A measuring instrument (such as a temperature sensor or an interferometer) is used to measure a value representing the expansion amount of the mask substrate. Alignment and positioning of the mask substrate and the photosensitive substrate is adjusted in response to the expansion amount of the reticle, based on the measured value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a mask substrate used in a lithographic processfor manufacturing, e.g., semiconductor devices or liquid crystal displaydevices, and to a projection exposure apparatus that transfers a circuitpattern onto a photosensitive substrate, such as, e.g., a semiconductorwafer or a glass plate, from a mask substrate. The invention alsorelates to a method of forming a circuit pattern onto a photosensitivesubstrate using a projection exposure apparatus.

2. Description of Related Art

In manufacturing semiconductor devices (e.g., VLSI) or liquid crystaldevices, it is indispensable to perform a lithographic process forexposing and transferring a circuit pattern onto a photosensitivesubstrate, such as a photoresist-coated semiconductor wafer or glassplate. The lithographic process uses a projection exposure apparatus toproject and expose a circuit pattern image formed on a mask substrate(also referred to as a reticle) onto a photosensitive substrate througha projection optical system having a magnification equal to or lessthan 1. This projection exposure apparatus is called a stepper, becauseit operates in a step-and-repeat manner to repeat an action of driving atwo-dimensional driving stage supporting a photosensitive substrate by apredetermined amount to a next shot area for exposure, every timeexposure of the projection image of the reticle circuit pattern onto ashot area on the photosensitive substrate has been accomplished.

In order to simultaneously achieve high resolution and a broaderexposure field, a step-and-scan type projection exposure apparatus hasrecently been proposed. In this technique, the reticle and thephotosensitive substrate are scanned one-dimensionally relative to thefield of view of the projection optical system during exposure of oneshot. When exposure is not performed, the photosensitive substrate isdriven in a stepwise manner. See, e.g., "Optical/Laser MicrolithographyII", SPIE Vol. 1088, at 424-433 (1989). Also see, e.g., U.S. Pat. No.5,477,304 and U.S. Pat. No. 4,924,257, both of which are incorporatedherein by reference in their entireties.

The reticle is fixed to and supported on the reticle stage in theapparatus by applying a vacuum thereto, so that the main surface of thereticle, on which the circuit pattern is formed, is precisely alignedwith the object surface of the projection optical system. Byilluminating the pattern area (normally, a rectangular area ofillumination is used, although, as disclosed in the above-incorporatedpatents, the illumination area also can be arcuate or hexagonal, forexample) of the reticle mounted on the stage with exposure illuminationlight, the pattern image formed on the pattern area is projected throughthe projection optical system and onto the photosensitive substrate.

Generally, the reticle is formed by etching a light-blocking material(e.g., chromium) layer formed on the main surface of a quartz plate byevaporation into the circuit pattern. Alternatively, a certain type ofphase shift reticle is formed by etching a transmissive material shifterlayer formed on the main surface of the quartz plate into the circuitpattern.

The size of the reticle has increased from 4 inches, to 5 inches, andcurrently to 6 inches as a standard, as integration techniques haveimproved and as the device (i.e., the chip) size has increased. In anexposure apparatus for producing liquid crystal devices in which thecircuit pattern is exposed by a scanning scheme using a projectionoptical system with a magnification of 1, the mask and thephotosensitive substrate (glass plate) are the same in size, and a masklarger than 40×40 cm may be used.

In the manufacturing site of semiconductor devices, mass production of64M D-RAM has started. Moreover, although still in the trial stage ofmanufacturing, a great deal of study and development for mass productionof 256M D-RAM and 1 G D-RAM has been made. It is expected that massproduction of both 256M (Megabyte) memory and 1 G (Gigabyte) memorydevices will require a projection exposure apparatus that uses anultraviolet light source.

On the other hand, the accuracy standard has become more and more strictin various functions for manufacturing such devices required in theprojection exposure apparatus. In particular, very strict precision inimage formation of the projection optical system and alignment accuracybetween the reticle and the photosensitive substrate (wafer) arerequired. For this reason, a design and manufacturing method forapproaching an ideal image formation capacity of the projection opticalsystem has been desired. Developing various types of sensors is alsodesired to improve the alignment precision.

However, even though efficiency and performance are improved in theprojection exposure apparatus, a problem of heat energy accumulationstill remains, which is caused by continuously illuminating the reticlewith illumination light. The extent of heat accumulation variesdepending on the material used to make the circuit pattern formed on thereticle and the light transmittance through the reticle. If the circuitpattern is formed with a non-transmissive material layer, and if theratio of the total light-blocking portion to the overall illuminatedarea is greater, heat accumulation becomes great.

Due to such heat accumulation, the temperature of the reticle rises, andas a result, the reticle slightly expands. Since the four corners of thereticle are fixed to the reticle stage at the vacuum mounting positions,the reticle is physically warped by the heat expansion. This adverselyaffects the flatness of the reticle. If the flatness of the reticle isdegraded, the projected pattern image will contain errors, such asdistortion aberration, image surface distortion, or image surfacetilting, even if the imaging property of the projection optical systemis close to ideal. This error adversely affects the final image of thecircuit pattern to be transferred onto the photosensitive substrate.

In particular, a problem arises from the fact that heat accumulation andheat release of the reticle slightly varies depending on the change inthe exposure sequence of the photosensitive substrate and theillumination conditions. More seriously, a reticle may be replaced withanother reticle having a completely different circuit pattern, andtendencies of heat accumulation and heat release are different for eachreticle. Thus, although the same projection exposure apparatus is used,the quality of the image to be transferred onto the photosensitivesubstrate changes depending on the circuit pattern.

Even in a single reticle, the influence of heat accumulation is totallydifferent, for example, between a situation where the reticle has justbeen mounted on the exposure apparatus, and a situation where exposureoperation has been continuously performed. Consequently, the quality ofthe transferred pattern image deteriorates gradually.

SUMMARY OF THE INVENTION

Therefore, it is an object of embodiments of the present invention toreduce the deterioration of the projected image due to a slightexpansion of the reticle (mask substrate) that occurs in a projectionexposure apparatus.

It is also an object of embodiments of the invention to provide aprojection exposure apparatus that is capable of accurately correctingan alignment error and a superposing error, caused by minute positionalshifts of the reticle itself on the reticle stage in addition to aslight expansion of the reticle.

It is a further object of embodiments of the invention to provide animproved mask substrate suitable for the projection exposure apparatus.

It is another object of embodiments of the invention to provide a methodfor forming a circuit pattern while preventing the transferred image(e.g., the entire rectangular pattern area) of the mask substratecircuit pattern onto the photosensitive substrate from slightly changingin size due to expansion of the mask substrate.

One aspect of the invention relates to a mask substrate comprised of atransparent parallel plate (e.g., a quartz plate) having a giventhickness. A circuit pattern, which is to be transferred onto thephotosensitive substrate, is formed, e.g., from a light-blockingmaterial layer or a phase-shifter material layer on one of the mainsurfaces of the transparent parallel plate. The periphery of thetransparent parallel plate is defined by, e.g., four side surfaces.Reflective areas are formed on at least a portion of the side surfacesof the transparent parallel plate with a reflective material (e.g.,metal, such as chromium and aluminum, or a dielectric film) having agiven reflectance to a light beam. The reflective areas on the sidesurfaces are used as reference positions when the circuit pattern isformed on the main surface of the parallel plate. In other words, thecircuit pattern is printed on the mask substrate with precisepositioning using the reflective portions of the side surfaces asreference marks. Accordingly, by monitoring the positions of thereflective surfaces when the reticle is mounted on the projectionexposure apparatus, the position of the reticle pattern can be preciselyknown.

A second aspect of the invention relates to a projection exposureapparatus that is comprised of an illumination system for illuminating amask substrate having a circuit pattern thereon with exposureillumination light, a projection optical system for projecting thecircuit pattern image of the mask substrate onto a photosensitivesubstrate by means of illumination with the illumination light, and atwo-dimensional driving stage for supporting the photosensitivesubstrate and two-dimensionally moving the photosensitive substraterelative to the field of view of the projection optical system. The masksubstrate, on which the circuit pattern is formed, is supported so thatthe main surface of the mask substrate is aligned with the object planeof the projection optical system. To achieve this alignment, theprojection exposure apparatus includes a mask stage, a column structure,a plurality of interferometers and a driving mechanism. The mask stageincludes a plurality of mounts for contacting the periphery of the masksubstrate at a plurality of positions to support the mask substratewhile allowing free expansion of the mask substrate on the mounts alongthe main surface. The column structure supports the mask stage so thatthe mask stage moves within a plane substantially parallel to the mainsurface of the mask substrate. The plurality of interferometers measurepositional shift information of the reflective areas formed on at leasta portion of the side surfaces of the mask substrate by emittingmeasuring beams to the reflective areas and by receiving the beamsreflected by the reflective areas. The driving mechanism moves the maskstage relative to the column structure (or moves the mask relative tothe mask stage) based on the information measured by theinterferometers.

A third aspect of the invention also relates to the projection exposureapparatus. In order to support the mask substrate having the circuitpattern thereon so that the main surface of the mask substrate isaligned with the object plane of the projection optical system, theprojection exposure apparatus comprises a mask stage having a pluralityof mounts for contact with the periphery of the mask substrate at aplurality of positions to support the mask substrate while allowingminute movement along the main surface on the mounts. The columnstructure integrally supports the mask stage with regard to theprojection optical system. The plurality of interferometers measurepositional shift information of the reflective areas formed on at leasta portion of the side surfaces of the mask substrate by emittingmeasuring beams to the reflective areas and by receiving the beamsreflected from the reflective areas. The driving mechanism slightlymoves the mask substrate on the mask stage based on the informationmeasured by the interferometers.

A fourth aspect of the invention relates to a method for forming acircuit pattern on a photosensitive substrate by repeating exposureoperations in which illumination light is illuminated on a pattern areaformed on a mask substrate to successively expose a pattern image in thepattern area onto each of a plurality of shot areas on thephotosensitive substrate through a projection optical system. The masksubstrate is positioned on the object plane side of the projectionoptical system so as to allow free expansion of the mask substrate. Anexpansion amount of the mask substrate relative to an initial state isdetected, the expansion being caused by heating of the mask substrateduring the repeating exposure operations using the mask substrate. Thiscan be accomplished, e.g., by measuring with a temperature sensor or aninterferometer system based on the side surfaces of the mask substrate.Then, an image formation characteristic of the projection optical systemis adjusted in response to the detected expansion amount to correctslight changes caused by the expansion of the mask substrate in thestate of the pattern image to be exposed onto the shot areas of thephotosensitive substrate. This can be accomplished, e.g., by moving aportion of the optical lens group forming the projection optical system,adjusting the gas pressure of an air interval between lenses, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the followingdrawings in which like reference numerals designate like elements andwherein:

FIG. 1 is a schematic diagram showing the overall structure of theprojection exposure apparatus in accordance with a first preferredembodiment of the invention;

FIG. 2 is a perspective view showing the positional structure of thereticle stage and the interferometer system according to the firstembodiment of the invention;

FIG. 3 is a plan view of the reticle stage of FIG. 2;

FIG. 4 is a block diagram showing the structure of the reticle stagecontrol system according to the first embodiment of the invention;

FIG. 5 is a perspective view showing the structure of the reticle stageaccording to a second embodiment of the invention;

FIG. 6 is a perspective view showing the structure of the reticle stageaccording to a third embodiment of the invention;

FIG. 7 is a perspective view showing the structure of the reticle stageaccording to a fourth embodiment of the invention;

FIG. 8 is a plan view showing the structure of the interferometer systemfor expansion amount measurement in accordance with a fifth embodimentof the invention;

FIG. 9 is a schematic side view showing the structure of the projectionexposure apparatus in accordance with a sixth embodiment of theinvention;

FIG. 10 is a perspective view showing the structure of the sample stageof the electron beam (EB) exposure apparatus that executes the reticlemanufacturing method in accordance with a seventh embodiment of theinvention;

FIG. 11 is a plan view showing the structure of the reticleinterferometer system in accordance with an eighth embodiment of theinvention; and

FIG. 12 is a perspective view showing the structure of the reticle stageof the projection exposure apparatus in accordance with a ninthembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the invention, a reflective area is formed on atleast a portion of the side surfaces of a mask substrate (otherwiseknown as a reticle) R. A measuring beam is emitted from aninterferometer to the reflective area to measure the positional shift ofthe mask substrate. At the same time, the mask substrate is supported onthe mask stage by vacuum so as to allow for the free expansion of themask substrate without being rigidly fixed thereto. Due to thisstructure, distortion of the projected image due to slight expansion ofthe mask substrate is greatly reduced, while the accurate monitoring ofthe positional shift of the circuit pattern image caused by the slightexpansion of the mask substrate is permitted.

Because the position of the side surface of the mask substrate isdirectly measured by the interferometer, and because the mask substrateis driven based on the measured value, moving mirrors fixed to the maskstage and used in the conventional system for monitoring the position ofthe mask stage, can be omitted. In the conventional system, in spite ofthe fact that the position of the mask substrate supported on the maskstage is slightly changed by vibrations acting on the exposure device,the positional change was not clearly measured, which worsened thealignment error and superposition error arising during exposure.

On the contrary, in the present invention, the mask substrate issupported on the mask stage by a soft friction or soft pushing forceapplied perpendicularly to the stage. Basically, the positional shift ofthe mask substrate on the mask stage does not become a main cause of thealignment error and superposition error, because the positional shift isdirectly detected by the interferometers as a shift measurement. Thequality of the projected image is not adversely affected even if themask substrate slightly expands and slightly slips out of position. Thiscan be realized in mass production of microscopic devices with a lineinterval less than 0.25 μm, such as 256M memories or 1 G memories, witha high accuracy and yield rate.

The structure of the projection exposure apparatus of a first embodimentof the invention will be described, making reference to FIGS. 1, 2 and3. FIG. 1 schematically shows the overall structure of a step-and-repeattype projection exposure apparatus. The illumination optics (IL) emitsexposure illumination light to illuminate the circuit pattern area ofthe reticle R with uniform intensity distribution. The reticle R ismounted on the reticle stage RST through four mounts 10 that project atfour positions on the reticle stage RST. Among the four mounts 10, oneor two of these mounts 10 support the reticle R by applying a vacuumthereto.

Alignment system 20A and 20B photoelectrically detect the alignmentmarks provided at two points on the periphery of the reticle R, therebypositioning the reticle R with regard to the optical axis AX of theillumination optics IL and the projection optical system PL with apredetermined precision. The positioning is carried out by a drivingsystem 12 that translates the reticle stage RST within the X-Y plane,which is perpendicular to the optical axis AX, while slightly rotatingthe reticle stage RST within the X-Y plane.

The reticle stage RST is movably supported on the reticle stage basestructure CL1, which constitutes a part of the column structure of theapparatus body. The driving system 12, including its motor, is alsomounted on the base structure CL1. The beam interference elements (i.e.,the beam splitter, etc.) of the reticle interferometer system IFR, whichis one feature of the invention, is also fixed onto the reticle basestructure CL1. The interferometer system IFR emits measuring beam BMrperpendicularly to the reflective area formed on a portion of the sidesurface of the reticle R, and receives the reflected beam to detect thepositional shift of the reticle R.

The circuit pattern image of the reticle R is projected and focused ontothe wafer W, which is a photosensitive substrate, with a magnificationof, e.g., 1/4 or 1/5, through the projection optical system PLpositioned directly under the reticle stage RST. The lens barrel of theprojection optical system PL is fixed to the lens base structure CL3,which constitutes a part of the column structure. The lens basestructure CL3 supports the reticle base structure CL1 through aplurality of columns CL2.

The reticle interferometer system IFR shown in FIG. 1 is designed sothat the reflected beam of the measuring beams BMr interferes with thereference beam reflected from the reference mirror FRr fixed above theprojection optical system PL. However, as alternatives, the referencemirror may be fixed to the reticle base structure CL1, or may be builtinto the interferometer system.

The lens base structure CL3 is mounted on the wafer base structure CL4.The wafer base structure CL4 houses the wafer stage WST, which supportsthe wafer W and two-dimensionally moves along the X-Y plane. The waferstage WST is equipped with a wafer holder that holds the wafer W byapplying a vacuum thereto so that the surface of the wafer W is inalignment with the image formation plane of the projection opticalsystem PL. Wafer stage WST also is equipped with a leveling table forslightly moving the wafer holder in the Z direction (i.e., along theoptical axis AX) while slightly slanting the wafer holder.

The coordinate position and a slight rotation amount generated by yawingof the wafer stage WST within the X-Y plane are measured by the waferinterferometer system IFW. The wafer interferometer system IFW detectsthe coordinate position and the rotation amount (yawing amount) of thewafer stage WST by emitting a laser beam from a laser light source LS tothe moving mirror MRw attached to the leveling table of the wafer stageWST and to the fixed mirror FRw fixed to the lowermost position of theprojection optical system PL, and by having the two reflected beams fromthe respective mirrors MRw and FRw, respectively, interfere with eachother.

A reference plate FM is also mounted on the leveling table of the waferstage WST. The reference plate FM is used for calibration and base linemeasurement (i.e., measurement of the positional relationship betweenthe projection point of the center of the reticle pattern and thedetection center of each of the alignment systems) of various alignmentsystems, focus sensors and leveling sensors. On the surface of thereference plate FM, a reference mark is formed, which is detectabletogether with a reference mark on the reticle R by the alignment systems20A and 20B under illumination light having an exposure wavelength.

The base line measurement using a reference plate FM is disclosed in,for example, Japanese Laid-Open Patent Application No. 4-45512 andJapanese Laid-Open Patent Application No. 5-21314, the disclosures ofwhich are incorporated herein by reference in their entireties. A focuscalibration method using a reference plate FM is disclosed in, forexample, these Laid-Open Japanese Patent publications. Therefore, adetailed explanation thereof will be omitted here.

FIG. 2 is a perspective view of the reticle stage RST of FIG. 1 and thesurrounding elements. In this figure, the reticle R is correctlypositioned with respect to the optical axis AX of the projection opticalsystem PL. An opening 11 is formed in the center of the reticle stageRST so as not to block the projected light paths through the rectangularpattern area PA formed on the reticle R and the two reference marks M1,M2 formed on either side of the pattern area PA in the X-axis direction.Mounts 10A, 10B, 10C and 10D are formed on the reticle stage RST aroundthe opening 11 at four positions so as to have a predetermined height(10C and 10D are not seen in FIG. 2 because they are located behind thereticle R). These mounts support the reticle R on its rear side (i.e.,the main surface on which the pattern is formed) at four cornersthereof.

The driving system 12 for the reticle stage RST includes three drivingunits 12A, 12B and 12C. The driving unit 12A slightly drives the reticlestage RST in the X direction. The other two driving units 12B and 12Ccooperate to slightly drive the reticle stage RST in the Y directionwhile slightly rotating it. The driving units 12A, 12B and 12C can be,e.g., actuators that linearly push the corresponding positions on thereticle R. Springs 13 are provided at several positions between thereticle stage RST and the reticle base column CL1 to force the reticlestage RST toward the direction of the actuators.

As shown in FIG. 2, reflective areas SX, SY and Sθ, which receivemeasuring beams from the reticle interferometer system, are formed ontwo orthogonal side surfaces of the reticle R. The reflective areas SX,SY and Sθ are formed by forming a reflective material layer, such as analuminum layer, or a dielectric film having a high reflectance to thewavelength of the measuring beams, by evaporation after grinding theside surfaces of the reticle R. Although, in FIG. 2, the reflectiveareas SX, SY and Sθ are provided on a portion of the side surfaces ofthe reticle R, the entire area of a side surface extending in the Xdirection and the entire area of a side surface extending in the Ydirection may be reflective.

The reticle interferometer system IFR includes an X-directioninterferometer IFRX including a reflective mirror BR1 for reflecting themeasuring beam BMrx in the X direction toward the reflective area SX, aY-direction interferometer IFRY including a reflective mirror BR2 forreflecting the measuring beam BMry in the Y direction toward thereflective area SY, and a Y-direction interferometer IFRθ including areflective mirror BR3 for reflecting the measuring beam BMrθ in the Ydirection toward the reflective area Sθ.

The measuring beam BMrx reaching the reflective area SX is projected soas to be perpendicular to the optical axis AX of the projection opticalsystem PL, while the measuring beams BMry and BMrθ reaching thereflective areas SY and Sθ, respectively, are emitted so as to beparallel to each other with a predetermined distance LK between them inthe X direction. The positional shift of the reticle R in the Xdirection is detected by the measurement value of the interferometerIFRX, and the positional shift in the Y direction is detected bycalculating the average of the measurement values of the interferometersIFRY and IFRθ. The positional shift in the rotational (θ) direction ofthe reticle R is detected by calculating the difference between themeasurement values of the interferometers IFRY and IFRθ.

The measuring resolution of the interferometers IFRX, IFRY and IFRθ isselected so that the expansion amount of the reticle R, which is definedby the linear expansion coefficient of the reticle R, the expectedtemperature changing range, and the size (e.g., 6 inches) of the reticleR, can be detected. The profiles of the measuring beams BMrx, BMry andBMrθ at the side surfaces of the reticle R are shaped in an ellipseflattened along the side surface, or in a slit-like shape. The profilesof each of the reference beams emitted toward the reference mirrors FRrfixed at the top of the projection optical system PL are also shaped inan ellipse flattened in the same direction as the measuring beams, or ina slit-like shape.

The reason why the profile of the measuring beams and of the referencebeams are shaped in an ellipse or slit is to prevent deviation from theinterference state, which is caused by the fact that the advancingdirection of the measuring beam reflected from the reticle side surfaceis slightly deflected by the slight rotation of the reticle R within theX-Y plane, within the rotation range where the reticle R is present.

FIG. 3 is a plan view of the reticle stage RST shown in FIG. 2. Themounts 10A, 10B, 10C and 10D for supporting the reticle R are providedat four corners around the opening 11, which is formed so as not toblock the projected light path through the pattern area PA and the marksM1 and M2. The adsorption (contact) surface of the mount 10C, which islocated farthest from either of the reflective areas SX, SY and Sθilluminated by the measuring beams BMrx, BMry and BMrθ of the reticleinterferometer system, is formed larger than the adsorption surfaces ofthe other three mounts 10A, 10B and 10D, to increase the vacuumadsorption thereof.

Ports 15A, 15B, 15C and 15D are provided at the periphery of the reticlestage RST and function to connect the adsorption surfaces of the mounts10A, 10B, 10C and 10D to a vacuum source. By applying a predeterminedvacuum power to each of the ports 15A through 15D, the four corners ofthe reticle R can be rigidly adsorbed to (held in contact with) thereticle stage RST. However, in the preferred embodiment, only the mount10C strongly adsorbs the reticle R, while releasing the vacuumadsorption to the other mounts to allow the reticle R to be supported byits own weight, at least during the successive exposure of a pluralityof wafers W.

Supporting the reticle R only at the mount 10C by vacuum adsorptionallows the reticle R to freely expand along the direction of thereflective areas SX, SY and Sθ formed on the two orthogonal sidesurfaces, with the mount 10C being used as a base point, as thetemperature changes. This structure can prevent stress from beinggenerated in the reticle R as the reticle R expands, and allowssubstantially linear expansion along the X-Y plane. In response to theamount of free expansion of the reticle R, the positions of thereflective areas SX, SY, Sθ slightly change along the X and Ydirections. The amounts of the positional shift of the respectivereflective areas caused from the free expansion are measured by thereticle interferometers IFRX, IFRY, and IFRθ with measuring beams BMrx,BMry, and BMrθ, respectively.

There may be a situation where the reticle R itself is slightlydisplaced from the reticle stage RST in spite of the strong adsorptionof the mount 10C. If this is the case, the reticle interferometers IFRX,IFRY and IFRθ will output the measurement values containing both thepositional shift of the reticle R itself and the heat expansion.Therefore, calculating positional shift (in the X and Y directions) androtational displacement of the center point CC of the reticle R from theinitial state based on only the measurement values from the reticleinterferometers IFRX, IFRY and IFRθ cannot precisely determine whetherthe positional shift and the rotational displacement are caused byexpansion of the reticle R or by the shifting of the reticle R itself,or by both factors.

Accordingly, because accurate information is lacking about the amount ofpositional shift and rotational displacement calculated from only themeasurement values of the reticle interferometers, it is not reliablefor performing feedback control of the driving units 12A, 12B and 12Cfor the reticle stage RST without additional information.

To overcome this problem, a temperature sensor system is provided forthe projection exposure apparatus. The temperature sensor system detectsthe temperature of the reticle R with high accuracy and can be used toaccurately specify the linear and rotational displacement of the centerpoint CC of the reticle R from the initial position. When thermoelectriccouple-type sensors are used in the temperature sensor system, amechanism is provided for bringing the sensors into contact with severalpositions of the reticle R except for the pattern area PA and the markareas M1, M2. When a non-contact type temperature sensor (e.g., anIR-CCD camera, etc.) is used, an optical system is provided fordetecting the temperature change (and temperature distribution) of theentire surface of the reticle R through a condenser lens system withinthe illumination optics IL (of FIG. 1).

Based on the linear expansion coefficient of the transparent plate(e.g., the quartz plate) of the reticle R, which is determined inadvance, the measured temperature change from the reference temperature(e.g., a predetermined temperature for manufacturing the reticle R), andthe distance (positional relationship) between the support point (mount10C) on the reticle R and each of the reflective areas SX, SY and Sθ,the positional shift amount Δmx, Δmy and Δmθ of the reflective areas SX,SY and Sθ due to the expansion component of the reticle R is estimatedfirst. Based on the results of this estimation, the positionaldisplacement of the center point CC (ΔXt, ΔYt), and the rotationaldisplacement of the reticle (ΔRt) due to expansion, are determined.

Then, the positional change amount ΔFx, ΔFy and ΔFθ from the initialposition of the reflective areas SX, SY and Sθ, which was actuallymeasured by the reticle interferometers IFRX, IFRY and IFRθ, are readout to calculate deviation values ΔMx, ΔMy and ΔMθ from the estimatedpositional shift amount Δmx, Δmy and Δmθ as follows:

ΔMx=ΔFx-Δmx (X-direction component)

ΔMy=ΔFy-Δmy (Y-direction component)

ΔMθ=ΔFθ-Δmθ (Y-direction component)

If the deviations ΔMx, ΔMy and ΔMθ are within an acceptable range andare close to zero (0), it is known that the reticle R itself has notslipped out of position on the reticle stage RST. Therefore, the drivingunits 12A, 12B and 12C are feedback controlled so as to correct only theestimated linear displacement amount (ΔXt, ΔYt) and rotationaldisplacement (ΔRt) due to the reticle expansion.

If the deviations ΔMx, ΔMy and ΔMθ are out of the acceptable range, itis regarded that the deviations are caused by the positional shifting ofthe reticle R itself. In this case, based on the deviations ΔMx, ΔMy andΔMθ, the displacement amount (ΔXs, ΔYs) and the rotational displacement(ΔRs) of the center point CC, due to the positional shift of the reticleR, are calculated as follows:

ΔXs=ΔMx

ΔYs=(ΔMy+ΔMθ)/2

ΔRs=(ΔMy-ΔMθ)/Lk,

where Lk is a distance along the X direction between the measuring beamsBMry and BMrθ from the reticle interferometers (shown in FIG. 3). Theestimated displacement (ΔXt, ΔYt) and rotational displacement (ΔRt) dueto expansion are added to the linear displacement (ΔXs, ΔYs) androtational displacement (ΔRs) due to the positional shift of the reticleR to obtain the final linear displacement (ΔXs+ΔXt, ΔYs+ΔYt) androtational displacement (ΔRs+ΔRt) of the center point CC of the reticleR. The driving units 12A, 12B and 12C are feedback controlled based onthe measurement values of the interferometers IFRX, IFRY and IFRθ so asto correct the linear displacement of the center point CC and therotational displacement.

FIG. 4 is a schematic block diagram showing the structure of the controlsystem for performing the calculation and control operations describedabove. The reticle interferometers IFRX, IFRY and IFRθ include receivers21X, 21Y and 21θ, respectively, for photoelectrically detectinginterference beams between the reflected measuring beams from thereflective areas SX, SY and Sθ, respectively, and the reflectedreference beams from the reference mirrors.

The signals from the receivers 21X, 21Y and 21θ are input to the countercircuit units 22X, 22Y and 22θ, respectively, which output digitalvalues corresponding to the position of the measuring direction andpositional change of the reflective areas SX, SY and Sθ in real time.The digital values are input to the CPU 24, which then calculatespositional errors, such as linear displacement and rotationaldisplacement, of the center point CC of the reticle R, and outputscontrol information for correcting the errors to the interface circuit26. The interface circuit 26 outputs an optimum control command value toeach of the servo circuits 28A, 28B and 28C, which drive the motors ofthe three driving units 12A, 12B and 12C, respectively.

The control system in FIG. 4 also includes temperature sensors 30A and30B that detect temperature change at a plurality of points on thereticle R in a non-contact manner. The signals from the temperaturesensors 30A and 30B are converted to digital values by the temperaturemeasuring circuits 31A and 31B, respectively, and the digital outputsare input to the CPU 24. Based on the temperature information, the CPU24 reads out the expansion coefficient of the reticle R and dataindicative of the positional relationship among the reflective areas SX,SY and Sθ on the reticle R, which are stored in the memory in advance,to estimate the positional shift amounts Δmx, Δmy and Δmθ of thereflective areas SX, SY and Sθ, respectively, due to the expansion ofthe reticle R in the measuring direction.

In this embodiment, vacuum adsorption is applied only to the mount 10Cto adsorb and fix the reticle R to the reticle stage RST. However, theinvention is not limited to this structure. Soft vacuum adsorption mayalso be applied to the other three mounts 10A, 10B and 10D as long as itdoes not prevent the free expansion of the reticle R. Alternatively,soft vacuum adsorption may be uniformly applied to all four of themounts 10A, 10B, 10C and 10D in a range that does not prevent freeexpansion of the reticle R.

Since the reticle stage RST fixedly adsorbs the reticle R through themount 10C at one corner, or loosely supports the reticle R through thefour mounts with soft vacuum adsorption, the reticle R can freely expandin the X-Y plane direction in response to the temperature change withoutgenerating undesirable stress. This prevents deformation of the patternsurface of the reticle R, and therefore, prevents the projected imagefrom being gradually deteriorated during the continuous exposureprocess.

A second embodiment of the invention will now be described withreference to FIG. 5. FIG. 5 shows a modification of the reticle stageRST of FIG. 1. The four corners of the reticle R are supported on thefour mounts 10A, 10B, 10C and 10D, similar to FIG. 3. However, in thisembodiment, the reticle R is mounted on the reticle stage only by itsown weight, without applying vacuum adsorption to the mounts, or withvery soft vacuum adsorption.

In this situation, movable pushers 36A, 36B, 36C, 36D and 36E positionedalong the periphery of the reticle stage RST softly push each of theside surfaces of the reticle R. When the reticle R is being transportedonto the mounts 10A-10D, the movable pushers 36A-36E are in a state ofbeing opened outward, as is shown by the movable pusher 36E in FIG. 5.When the reticle R is positioned on the mounts 10A-10D with a mechanicalpre-alignment precision (e.g., within ±1 mm), the five movable pushers36A-36E are raised vertically to push the corresponding side surfaces ofthe reticle R toward the center point CC of the reticle R. The reticle Ris positioned more precisely on the reticle stage RST by this gentlepushing force. The rotational positioning precision of the reticle R isset so that the measuring beams BMrx, BMry and BMrθ are reflected fromthe corresponding reflective areas SX, SY and Sθ on the side surface ofthe reticle at exactly the right angle to ensure accurate interferencemeasurement.

When the positioning has been accomplished, the movable pushers 36A-36Eare maintained at a slightly opened outward position so that elasticabutments 37A, 37B, 37C, 37D and 37E are in loose contact with thereticle side surfaces. The elastic abutments 37A-37E are formed on themovable pushers 36A-36E, respectively, so as to face the reticle sidesurfaces when the movable pushers 36A-36E are raised. If the reticle Rexpands due to, e.g., heat energy during a later process, the sidesurfaces of the reticle R will slightly deform the inner surfaces of theelastic projections 37A-37E outward.

When the reticle R is set on the reticle stage RST, the counter circuitunits 22X, 22Y and 22θ (FIG. 4) are reset to an initial value, and theCPU 24, interface circuit 26, and servo circuits 28A, 28B and 28Cimmediately start controlling the driving units 12A, 12B and 12C. Thereference mark on the reference plate FM formed on the wafer stage WST(FIG. 1) is positioned in the field of view of the projection opticalsystem PL. The alignment system 20A, 20B simultaneously detects themarks M1, M2 on the reticle R and the reference mark on the referenceplate FM to calculate the relative positional error and rotationaldisplacement (error), and sends a command to the CPU 24 to correct sucherrors.

In response to the command, the driving units 12A, 12B and 12C finelymove the reticle stage RST to achieve precise alignment between themarks M1, M2 of the reticle R and the reference mark on the referenceplate FM. When the precise alignment is accomplished, the driving units12A, 12B and 12C stop. At this point, the CPU 24 reads the measurementvalues of the counter circuit units 22X, 22Y and 22θ and stores them asinitial values. The initial values represent the position of thereflective areas SX, SY and Sθ of the reticle R before expansion.

FIG. 6 shows the reticle R and the reticle stage in accordance with athird embodiment of the invention. The reflective areas are formed onthe entire surface of the four side surfaces of the reticle R. Theexpansion amount of the reticle R, caused by temperature change, isdirectly measured by a dedicated interferometer system designed so as todirectly measure the change in the distance between two parallel sidesof the reticle R. That is, the change in reticle size in the X and/or Ydirections is detected.

One of the interferometer systems for expansion amount measurement willnow be described. A beam splitter BS1 splits a laser beam B0 into beamsBx and By. A mirror MRa reflects the beam By in the Y direction. A beamsplitter BSa receives the beam By from the mirror MRa and splits thebeam By into transmissive beams that advance to the side surface of thereticle R that extends in the X direction and into a reflected beam thatadvances toward the upper mirror MR2. A corner mirror MR4 receives thebeam BTy, which has been reflected by the mirror MR2 into the Ydirection parallel to the main surface of the reticle R, at the oppositeside of the reticle R, and reflects it to the side surface 39A of thereticle R at a right angle. A receiver 40Y receives the interferencebeam between the reflected beams, which has been reflected from the sidesurface 39A and returned to the beam splitter BSa through the cornermirror MR4 and mirror MR2, and the reflected beams, which have beenreflected from the side surface of the reticle R on the beam splitterBSa side and returned to the beam splitter BSa.

The information measured by the receiver 40Y represents a change inreticle size in the Y direction. The other interferometer system forexpansion amount measurement in the X-direction has the same structure.The beam Bx is reflected by the mirror MRb and is further split into twobeams by the beam splitter BSb. One beam is guided to the side surfaceof the reticle R extending in the Y direction, and the other beamreaches the side surface 39B of the opposite side of the reticle Rthrough mirror MR1 and corner mirror MR3. The two reflected beams fromtwo parallel side surfaces interfere with each other at the beamsplitter BSb, and the resultant interference beam is received at thereceiver 40X. The information measured by the receiver 40X representsthe change in reticle size in the X direction.

In this interferometer system, beam splitter BS1, mirrors MRa, MRb,MR1-MR4, beam splitters BSa, BSb (functioning as interference units),and receivers 40X, 40Y are basically built on the reticle stage shown inFIGS. 3 and 5. However, if the system is designed so that the incidentbeams Bx, By to the beam splitters BSa, BSb, respectively, are guided bya single mode optical fiber and the interference beams from the beamsplitters BSa and BSb are also guided by an optical fiber, then themirrors MRa, MRb, MR1-MR4, and beam splitters BSa, BSb are attached onthe reticle stage RST, and the other components, such as receiver 40X,40Y, may be attached to the reticle base column structure CL1.

The reticle R is mounted on the mounts 10A-10D of the reticle stage RST(FIGS. 3, 5) so as not to prevent free expansion of the reticle R. Afterthe reticle R is precisely positioned on the reticle stage RST using thealignment systems 20A, 20B (FIG. 1) and the reference plate FM, thecounter circuit that converts the measurement information from thereceivers 40X, 40Y into digital values is reset to start counting.

If the reticle R does not expand, the counter value of the countercircuit does not change. If the reticle R slightly expands, the countervalue changes from the initial value. By reading the change, theexpansion amounts in the X and Y directions are immediately obtained.Accordingly, the positional shift of the center point CC of the reticleR due to the expansion component can be accurately calculated, based onthe changed value.

If the projection exposure apparatus of FIG. 1 is equipped with a TTL(through-the-lens) alignment system that detects the mark on the wafer Wthrough only the projection optical system PL, or an off-axis type waferalignment system that is fixed outside the projection optical system PLand detects the mark on the wafer W (as disclosed in, for example,Japanese Laid-Open Patent Application No. 5-21314), it is necessary toprecisely measure the relative distance (base line) between theprojection point of the center CC of the reticle R and each of the markdetection center points of the TTL alignment system or off-axisalignment system, using a reference plate FM on the wafer stage WST,after the reticle R is mounted.

The base line measuring action is generally performed immediately aftera single reticle is mounted on the exposure apparatus. However, the baseline measuring action may be performed in an appropriate time intervalduring the continuous exposure process. If this is the case, the valuescounted in the counter circuit through the receiver 40X and 40Y are readout by the CPU 24 and are stored as initial values every time base linemeasurement is performed. Alternatively, the counter circuit connectedto the receivers 40X, 40Y may be reset every time base line measurementis performed.

According to the interferometer system for expansion amount measurementshown in FIG. 6, the expansion amount of the reticle R in the Xdirection and in the Y direction is directly measured, and the change inthe distance between the reticle center point CC and the side surfaces39A, 39B, to which the measuring beams BMrx, BMry and BMrθ are emittedfrom the interferometers IFRX, IFRY and IFRθ, is measured. Consequently,the positional change of the projected point of the reticle center pointCC is precisely determined in real time, and the alignment precisionbetween the reticle R and the wafer W is accurately controlled duringexposure.

FIG. 7 shows the reticle stage of a fourth embodiment of the invention.Unlike the embodiments described above, the reticle stage RST' of FIG. 7is fixed to the reticle column structure CL1 (FIG. 1), and moves betweenthe exposure position and the reticle receiving position only when thereticle is changed. The reticle R is mounted on the four mounts 10A,10B, 10C and 10D on the reticle stage RST'. Positioning of the reticle Ris performed by three linear actuators 50A, 50B and 50C (such as, e.g.,a piezoelectric device, a miniature linear motor, and a voice coil typemotor), which directly push the corresponding side surface of thereticle R. The reticle R used in this embodiment is one in which theentire area of the four side surfaces are polished (e.g., ground) toform reflective areas.

When the reticle R is mounted on the mounts 10A-10D with mechanicalpre-alignment, pusher 52A of the actuator 50A is driven in the Xdirection toward the side surface 39A of the reticle R, while pushers52B and 52C of the actuators 50B and 50C, respectively, are driven inthe Y direction toward the side surface 39B of the reticle R. On thecontact portion of each of the pushers 52A, 52B and 52C, which comesinto contact with the side surfaces of the reticle R, a chip made of,e.g., a steel ball or plastics (synthetic resins) is provided so as tominimize the friction coefficient.

When the two side surfaces 39A and 39B are pushed by the pusher 52A, 52Band 52C, reticle R finely moves in the X and Y directions, which causesthe opposite side surfaces 39C and 39D to contact the elastic abutments54A, 54B and 54C. The elastic abutment 54A makes point contact with theside surface 39D at the center thereof, and is elastically displaced inthe Y direction within a predetermined range (e.g., within severalmillimeters). The elastic abutments 54B and 54C make point contact withthe side surface 39C at both ends thereof, and are elastically displacedin the X direction within a predetermined range (e.g., within severalmillimeters). The elasticity of the elastic abutments 54A, 54B and 54Cis selected so as not to prevent free expansion of the reticle R.

The reticle R is finely moved in the X and Y directions by driving theactuators 50A, 50B and 50C, and the elastic abutments 54A, 54B and 54Care elastically displaced halfway in their available stroke. At thispoint, alignment of the reticle R is substantially accomplished withmechanical accuracy. Then, the actuators 50A, 50B and 50C are finelydriven so that the marks M1, M2 on the reticle R are aligned with thereference plate FM more precisely, using the alignment system 20A, 20Band the reference mark on the reference plate FM. As a result, thecenter point CC of the reticle R is precisely aligned with respect tothe coordinate system of the wafer stage WST.

Information about the expansion amount of the reticle R is provided bythe temperature measuring system shown in FIG. 4 or the interferometersystem of FIG. 6, similar to the first and third embodiments. Linear androtational displacement of the center point CC due to the expansion, aswell as the positional shift in the X and Y directions and therotational shift of the reticle R itself with respect to the mounts10A-10D, are corrected based on the measurement information. Wafer stageWST may be used to perform error correction in the X and Y directions,in addition to the actuators 50A, 50B and 50C positioned on the reticleside.

Since the driving power is directly applied to the side surface of thereticle R, the measurement surfaces that receive the measuring beamsBMrx, BMry and BMrθ from the reticle interferometers IFRX, IFRY andIFRθ, and the surfaces that contain a plurality of driving points in theactuators, are substantially the same. Consequently, control accuracy ofthe servo control system is improved without being subject to theinfluence of slip between the mounts 10A-10D and the reticle R.

FIG. 8 shows a modification in accordance with a fifth embodiment of theinvention in which a fiber interferometer system for directly measuringthe expansion amount of the reticle R is employed. In this embodiment, alight splitting area of dielectric film is formed on at least one sidesurface of the reticle R. The same numerals denote the same elements asin the previous embodiments.

The reticle R is mounted on four mounts 10A-10D on the reticle stageRST. Vacuum adsorption is applied to only one of the four mounts tosupport the reticle R, similar to the first embodiment. Reflective areaST is formed on the side surface 39A, in addition to the reflectiveareas SY and Sθ, to which measuring beams BMry, BMrθ are projected fromthe reticle interferometers IFRY, IFRθ. The reflective area ST is formedon a part of the side surface 39A that is polished by surface-grinding,and is used as a back reflector. Therefore, the reflective layer may bedeposited (i.e., via deposition) on the entire area of the side surface39A.

On the side surface 39C opposite the side surface 39A, a partialreflective area SS is formed with dielectric film. Expansion amountmeasuring beam BMt enters into the reticle R through the partialreflective area SS in parallel with the main surface of the reticle R.The measuring beam BMt is emitted to the partial reflective area SS onthe side surface 39C with a substantially normal incident angle througha single mode optical fiber 60, which receives a linear polarized beamB0 from the laser source. A metal fitting 62 fixes the exit end of theoptical fiber 60 to the reticle stage RST. A beam splitter 64 is fixedto the reticle stage RST and functions as an interference unit. A 1/4wave plate (e.g., a retardation sheet) is provided between the beamsplitter 64 and the side surface 39C, although it is not shown in thefigure.

Accordingly, the beam projected to the partial reflective area SSbecomes circular polarized light. A portion of the circular polarizedlight beam is reflected from the partial reflective area SS back to thebeam splitter 64 through the 1/4 wave plate, which makes the beam linearpolarized light perpendicular to the original light beam. The beamreflected by the beam splitter 64 enters the light-receiving opticalfiber 66, and is guided to a receiver (e.g., a photoelectric detectiondevice). On the other hand, the beam which has passed through thepartial reflective area SS becomes measuring beam BMt and reaches thereflective area ST on the opposite side surface 39A. The beam reflectedby the reflective area ST returns along the same light path as themeasuring beam BMt to the beam splitter 64 through the partialreflective area SS and the 1/4 wave plate. The beam is further reflectedby the beam splitter 64 and enters the optical fiber 66.

The reflected beam from the partial reflective area SS on the sidesurface 39C, and the reflected beams from the reflective area ST on theside surface 39A, interfere with each other to generate interferencebeam BF, which is received at the receiver. The receiver detects thechange in the Y direction, that is, the change in the distance betweenthe partial reflective area SS on the side surface 39C and thereflective area ST on the side surface 39A, with a resolution of lessthan 1/100 μm. The interferometer system comprised of beam splitter 64and receiver can directly measure the expansion amount of the reticle inthe Y direction.

The dielectric film (multi-layered film) that functions as a partialreflective area SS on the side surface 39C preferably has a reflectanceof 33% and a transmittance of 67%. Furthermore, since the beam B0 guidedto the beam splitter 64 is supplied from the optical fiber 60, it ispreferable to make the numerical aperture (N.A.) of the exit end of theoptical fiber 60 as small as possible to maintain the collimation of thebeam MBt. The system may be designed so that the beam B0 enters the beamsplitter 64 through a reflector fixed on the reticle base columnstructure CL1 (FIG. 1) without using the optical fiber 60.

FIG. 9 shows a projection exposure apparatus in accordance with a sixthembodiment of the invention. The beam splitter 64, which constitutes theexpansion amount measurement interferometer system of FIG. 8, is fixedto the reticle stage RST, and the interference beam between thereflected beam from the reflective area ST on the side surface 39A andthe reflected beam from the partial reflective area SS on the sidesurface 39C is received through the optical fiber 66 at the receiver 68.The signal from the receiver 68 is input to the processor unit 80, whichincludes a counter circuit and a CPU. The processor unit 80 calculatesthe expansion amount from the initial state of the reticle R in realtime. In this embodiment, a fine dimensional error (which will appear asa magnification error) in the circuit pattern image on the wafer W dueto expansion of the reticle R is corrected based on the measuredexpansion amount data.

To achieve this correction, the apparatus is designed so that the fieldlens G1 positioned in the telecentric unit of the projection opticalsystem PL can be translated, or moved with a tilt, in the direction ofthe optical axis AX. For such purpose, a plurality of piezoelectricdevices 70A, 70B for moving the field lens G1 are provided to theapparatus. The imaging magnification and distortion aberration of theprojection optical system PL can be delicately adjusted by controllingthe driving amount of the piezoelectric devices 70A, 70B using the drivecontrol circuit 72.

The focussing magnification of the projection optical system PL may befinely adjusted by sealing an appropriate air gap (air lens) 73 withinthe projection optical system and by adjusting the internal pressureusing a pressure control system 74. In order to correct the displacementof the optimum image formation plane in the optical axis (AX) direction,which is secondarily (derivatively) caused by the positional adjustmentof the field lens G1 and the pressure adjustment of the air lens 73, theapparatus is equipped with an oblique incident automatic focusing systemthat includes a detection system 75 for emitting imaging light obliquelyto the wafer surface and for receiving the reflected light FL to detectthe position of the wafer W in the optical axis direction, a pluralityof actuators 77 for moving the holder WH adsorbing the wafer W in the Zdirection, and a drive control system 76 for driving the actuators 77.Examples of oblique incident automatic focussing systems can be found inU.S. Pat. No. 4,558,949 and U.S. Pat. No. 5,448,332, the disclosures ofwhich are incorporated herein by reference in their entireties.

The processor unit 80 determines the enlarged or reduced scale (ppm) ofthe circuit pattern area on the reticle R with the center point CC as abase point, based on the measured expansion amount of the reticle R fromthe initial state, and outputs correction information to either thedrive control circuit 72 or to pressure control system 74 so that theerror (displacement) correction is executed corresponding to the scale.In response to the output, positional adjustment (correction) of thefield lens G1 or internal pressure adjustment (correction) of the airlens 73 is performed, and the dimensional error in the projected imageon the wafer W is corrected.

The drive control circuit 72 for controlling the position of the fieldlens G1, the pressure control system 74 for controlling the internalpressure of the air lens 73, and the drive control system 76, whichfunctions as an oblique incident automatic focusing system also have afunction for correcting the fluctuation in the image formation propertydue to change in the atmospheric pressure of the projection opticalsystem PL, due to partial absorption of the illumination energy duringexposure, or due to change in the condition of the illumination lightfrom the illumination optics for illuminating the reticle, as isdisclosed in Japanese Laid-Open Patent Application Nos. 60-78454 and62-229838, the disclosures of which are incorporated herein by referencein their entireties.

Since, in the embodiment, the expansion amount from the initial state ofthe reticle R is directly measured by the interferometer system frommoment to moment, apparent change in magnification due to the expansionof the reticle R can also be corrected in real time, whereby all typesof image forming errors occurring in the image formation light path(from the reticle pattern to the wafer surface) can be corrected.

In the embodiments described above, it is a main object to measure theexpansion amount of the reticle (mask substrate) due to heataccumulation and to correct alignment errors and projectionmagnification errors caused by such expansion. However, if a duct, forexample, is provided that can supply precisely temperature-controlledair to the reticle R on the reticle stage RST with a predetermined flowvelocity to sufficiently restrain the temperature rise due to heataccumulation, then temperature sensors for detecting the temperature ofthe reticle R, or an interferometer system for directly measuring theexpansion amount, may be omitted.

However, even in that situation, it is still preferable to detect thedisplacement amount of the reticle R in the X, Y and θ directions usingthe reflective area (mirror surface) formed on the side surface of thereticle R and interferometers IFRX, IFRY and IFRθ, and to finely adjustthe reticle R based on the measurement information. The conventionalapparatus that has a reflector fixed to the reticle stage RST for finemovement cannot detect microscopic slip (positional shift ordisplacement) of the reticle R itself, as mounted on the reticle stageRST.

In the above embodiments, at least a portion of the side surfaces of thereticle R (mask substrate) is polished by a mirror grinding technique,onto which a reflective layer is formed of reflective materialdeposition. The reflective layer is preferably formed before the circuitpattern is printed on the quartz substrate as a material of reticle R.

Generally, in the manufacturing process of reticle R, the quartzsubstrate (which can be, e.g., 6 inches square with a thickness of 5mm), as the raw material, is optically ground so that the top and bottomsurfaces become parallel within an acceptable error range. Then, achromium layer having a low reflectance is deposited (e.g., byevaporation) on the entire area of one of the surfaces of the quartzsubstrate. This layer is further coated with a "resist" (e.g., aphotoresist) for use in electron-beam exposure (EB exposure). After thequartz substrate coated with resist is mounted on an EB exposure samplestage with mechanical pre-alignment, printing of the circuit pattern ormark pattern begins.

When the circuit pattern is exposed and projected onto the quartzsubstrate, the exposed portion of the photoresist is removed. Theremaining photoresist on the chromium layer forms a resist image. Theexposed substrate is further etched using the resist image as a masking,by a development process, thereby removing a portion of the chromiumlayer to make a reticle. The removed portion of the chromium layer willcorrespond to a transparent portion of the circuit pattern. In order toform a reflective area or partial reflective area on the side surface ofthe reticle R, all of the four side surfaces are ground during the stepof polishing the quartz substrate as a raw material, and a chromiumlayer (or a dielectric multi-layered film) is formed on all or some ofthe four side surfaces during the step of evaporating the chromium layeron the entire surface of the quartz substrate.

The sample stage of the EB exposure apparatus is equipped with referencepins that come into contact with two orthogonal side surfaces (e.g.,side surfaces 39A and 39B in FIGS. 6 and 8) of the quartz substrate toposition the quartz substrate on the sample stage. In this state, EBexposure is conducted to the resist on the quartz substrate that ispositioned on the sample stage. The circuit pattern and alignment marksto be printed are positioned using the two side surfaces as references.

The position of the circuit pattern is precisely defined by the two sidesurfaces. As a result, the reference point for pattern positioning forcompleting the reticle R is in agreement with the reference point formeasurement by the reticle interferometer system of the projectionexposure apparatus. The agreement of the reference points preventsalignment accuracy with the wafer W from being reduced due to theprinting error (positioning error) of the pattern on the reticle R.

A moving mirror is fixed to the periphery of the sample stage of the EBexposure apparatus, for use in the measuring system using laserinterferometers. A seventh embodiment relates to an improved samplestage of the EB exposure apparatus, which will be described below withreference to FIG. 10.

The quartz substrate QP is mounted on the sample stage 90. The EBexposure apparatus preferably includes interferometers EIX and EIY thatemit (project) measuring beams Bex and Bey to the side surfaces 39A and39B, respectively. Preferably, the position of the sample stage 90 isadjustably controlled using the side surfaces 39A and 39B of the quartzsubstrate QP as references, in response to the measuring results fromthe interferometers EIX and EIY.

More particularly, flatness in the side surfaces 39A and 39B of thequartz substrate QP in the extending direction is generally notpreferable. Therefore, position control of the sample stage 90 isperformed by the interferometer systems SIX and SIY using conventionalmoving mirrors MRwx and MRwy fixed on the sample stage 90, and theseparate interferometers EIX and EIY are used for measuring the flatnessof the side surfaces 39A and 39B.

In FIG. 10, the electronic lens system of the EB exposure apparatus hasan optical axis AX', and an electron beam (charged particle beam) PBF isemitted to the quartz substrate for printing the pattern. SA denotes ashot area that can be exposed by deflection of the beam PBF.

The sample stage 90 is linearly moved, for example, in the X direction,based on the coordinate values obtained by the interferometer systemsSIX and SIY, while successively sampling the measurement values of theside surfaces 39B detected by the interferometer EIY, thereby obtainingthe degree of flatness of the side surface 39B and the level to which itis parallel to the X axis. Similarly, as for flatness and deviation fromthe parallel plane to the X axis, the sample stage 90 is linearly movedin the Y direction based on the coordinate values from theinterferometer systems SIX and SIY, while successively reading themeasurement values from the interferometer EIX.

During actual printing of the pattern, print position errors arisingfrom the dispersion in flatness and deviation from the parallel plane ofthe side surfaces 39A and 39B is finely corrected by adjusting thetransporting position of the sample stage 90 and the deflecting positionof the exposure beam PBF. Thus, the circuit pattern is preciselypositioned and printed using the side surfaces 39A and 39B as referencemarks.

FIG. 11 is a plan view of the reticle stage R in accordance with aneighth embodiment of the invention. The reticle stage RST supports thereticle R with four mounts 10A-10D. The reticle stage RST is moved onthe reticle base column structure CL1 in the X, Y and θ directions bythe driving units 12A-12C. The position of the reticle R in the X and Ydirections is directly measured by the reticle interferometer system.

The reticle R used in this embodiment has cut-off side surfaces 103A and103B, which are formed by cutting off and polishing two corners of thereticle R diagonally (by 45°). Reflective layers are formed on the sidesurfaces that extend in the X and Y directions and define the other twocorners 105A and 105B of the reticle R. The interferometer system ofthis embodiment is designed so as to measure the displacement of thereticle R in the X and Y directions and rotation in the θ direction bydetecting positional shift components of the two corners 105A and 105Bof the reticle R in the direction of 45° from the X or Y axes.

In the first interferometer system, measuring laser beam LBa enters thebeam splitter 100A fixed to the reticle base column structure CL1 fromthe 45° angle direction with respect to the X-Y coordinate system. Thereflected component of the beam LBa reflected from the beam splitter100A is guided to the reference mirror 101A fixed to the reticle basecolumn structure CL1 at a right angle, and is reflected at a normalangle. The beam reflected by the reference mirror 101A passes throughthe beam splitter 100A and glass block (e.g., a rectangular solid) 102A.

On the other hand, the transmissive component of the beam LBa, which haspassed through the beam splitter 100A, enters into the reticle R fromthe diagonal (45°) side surface 103A in a direction parallel with themain surface of the reticle R, and is reflected by the rear surface ofthe reflective layer formed on the opposite corner 105A over twoorthogonal side surfaces. The reflected beam exits from the diagonal(45°) side surface 103A and reaches the total reflection surface 106A ofthe glass block 102A. The beam reflected by the total reflection surface106A traces the same light path in the opposite direction, exits fromthe diagonal side surface 103A, and is reflected by the beam splitter100A. This beam passes through the glass block 102A and is combined withthe other beam component reflected from the reference mirror 101A togenerate interference beam IBa.

The interference beam IBa is photoelectrically detected by the receiverof the first interference system, and the detection signal is connectedto the counter circuit of the later stage. As a result, the firstinterference system measures the positional shift of the corner 105A(the apex) of the reticle R in the diagonal (45°) direction with respectto the total reflection surface 106A of the glass block 102A.

The second interference system has a similar structure. The measuringbeam LBb is split by the beam splitter 100B into two components. Onebeam component is normally reflected by the reference mirror 101B andpasses through the beam splitter 100B and glass block 102B. The otherbeam component enters into the reticle R from the diagonal (45°) sidesurface 103B in a direction parallel to the main surface of the reticleR, and is reflected by the reflective layer formed on the oppositecorner 105B over the two orthogonal side surfaces. The beam exits fromthe diagonal (45°) side surface 103B and reaches the total reflectionsurface 106B of the glass block 102B.

The beam is further reflected by the total reflection surface 106B,traces the same light path in the opposite direction, and exits from theside surface 103B. The beam is reflected by the beam splitter 100B,passes through the glass block 102B, and is combined with the beamcomponent reflected from the reference mirror 101B to becomeinterference beam IBb, which is then received by the receiver. Thesecond interferometer system measures the positional shift of the corner105B of the reticle R in a diagonal (45°) direction, with respect to thetotal reflection surface 106B of the glass block 102B.

Two corners 105A and 105B of the reticle R work as corner mirrors, usingthe first and second interferometer systems. Therefore, even if there isa rotational shift of the reticle R within the X-Y plane, the positionof the reflected beams that return from the total reflection surfaces106A and 106B of the glass blocks 102A and 102B, respectively, do notshift.

Since displacement of two corners 105A and 105B of the reticle R aremeasured in the diagonal (45°) direction by the first and secondinterferometer systems, coordinate conversion is required to determinethe displacement in the X and Y directions. However, the advantage ofthis structure is that even when the reticle R is mounted on the reticlestage RST with a relatively large rotational shift (about 2-3°), theinterference systems, making use of the side surfaces of the reticle R,can normally function without reduction in precision.

Therefore, mechanical pre-alignment accuracy for initially mounting thereticle R on the reticle stage RST can be reduced, and pre-alignmentspeed can be increased. This results in reduced time in replacingreticle R.

A ninth embodiment of the invention will now be described with referenceto FIG. 12. FIG. 12 is a perspective view showing the structure of areticle base column structure CL1 and a reticle stage RST that aresuitable for a step-and-scan type projection exposure apparatus. Whenused in the step-and-scan type of apparatus, reticle R is mounted on thereticle stage RST of FIG. 12, which is one-dimensionally moved along theY direction with a stroke corresponding to the dimension of the reticleR (e.g. 6 inches) on the reticle base column structure CL1.

The one-dimensional movement is performed by the linear motor units 200Aand 200B that extend in the Y direction on both sides of the reticlestage RST. Each of the linear motor units 200A and 200B is entirelycovered with a housing. Air heated by the motor coil within the motorunits 200A, 200B is forcibly exhausted by the exhaust ducts 201A and201B.

Moving mirror 203 extending in the Y direction is fixed on one end ofthe X direction of the reticle stage RST. Three measuring beams BMrxfrom the reticle interferometer IFRX used for X direction measurement,the interferometer being fixed in base structure CL1, are verticallyprojected on the moving mirror 203. This interferometer IFRX performsmeasurement of minute positional changes and minute rotational errors ofthe X direction (non-scanning direction) of the reticle stage RST.Moreover, a plurality of meshlike openings 200C and 200D are formeddirectly under measuring beams BMrx in the cover of linear motor unit200B; whereby a flow caused through exhaust duct 201B of the airsurrounding measuring beams BMrx draws the heated air through eachopening 200C and 200D. Through this structure, and through the cover ofmotor unit 200B becoming warm, fluctuation of the light paths ofmeasuring beams BMrx is prevented.

Reticle R is positioned onto four mounts 10A-10D on stage RST (mount 10Cis hidden under reticle R, so it is omitted from FIG. 12). However, inthis preferred embodiment, in order to prevent the shifting of reticle Rtoward the Y direction (the scanning direction), due to the accelerationand deceleration of stage RST during scanning exposure, a protrusioncomponent is formed on one part of mounts 10B and 10D, so that the sidesurfaces of reticle R positioned diagonally will be pressured with agentle application of force along the Y axis. Moreover, in the preferredembodiment of this invention, all of the mounts 10A-10D maintain reticleR with gentle vacuum adsorption or, alternatively, any one of mounts 10Band 10D can maintain reticle R with strong vacuum adsorption.

At two locations on one side extending in the X direction of reticle R,notch corners CM1 and CM2 are formed at exactly 90° in the main surfaceof reticle R. Further, the slanted surfaces of each of the notch cornersCM1 and CM2 are optically ground and a reflective layer is deposited onthose surfaces. Accordingly, notch corners CM1 and CM2 operate as onetype of corner mirror, to reflect measuring beams BMry and BMrθ emittedfrom reticle interferometers IFRY and IFRθ, respectively, which arefixed in base column structure CL1 and are used for Y directionmeasurement. These measuring beams BMry and BMrθ irradiate one of theslanted surfaces of notch corners CM1 and CM2, respectively, and returnto interferometers IFRY and IFRθ from the other slanted surface parallelto the incident light path. As a result, each interferometer IFRY andIFRθ perform measurement of the position of the Y direction summit pointof each of the two corner parts CM1 and CM2.

In this way, corner parts CM1 and CM2 are constructed on one side ofreticle R, so that angle variation is prevented in each reflective beamof measuring beams BMry and BMrθ occurring through the rotation ofreticle R. A state of interference from each interferometer IFRY andIFRθ is allowed within the rotational range of reticle R, which is anadvantage of this preferred embodiment. So as to obtain this advantage,it is necessary that interval Lk, in the X direction of each measuringbeam BMry and BMrθ of interferometers IFRY and IFRθ, be in agreementwith the interval of the X direction of corner parts CM1 and CM2, whichare constructed in reticle R. This means that all reticles will have tobe prepared (by different manufacturers) with the same interval.However, if standards can be determined for quartz plates used for thistype of reticle, exposure device manufacturers can easily adapt to thisrequirement.

Stage RST, denoted in FIG. 12, is supported on base structure CL1through air-bearings, and is moved in a non-contact manner by linearmotor units 200A and 200B. Assuming that the position coordinates foreach Y direction of corner parts CM1 and CM2, measured by each of thetwo interferometers IFRY and IFRθ, are Yc1 and Yc2, then the rotationalvariable amount, Δθc, can be obtained from the initial state of reticleR, through the calculation (Yc1-Yc2)/Lk. The position coordinate Yr inthe Y direction of reticle R of the scanning exposure time can beobtained through the calculation (Yc1+Yc2)/2.

Based on this position coordinate Yr, linear motor units 200A and 200Bare controlled with identical driving force, so that reticle stage RSTis moved with a prescribed speed in the Y direction. Additionally, bycreating a difference between the driving force of linear motor units200A and 200B, so that the measured rotational variable amount Δθcbecomes an invariably fixed value (i.e., zero), or, alternatively,becomes a value that varies according to a particular tendency, minuterotations may be made to reticle stage RST. In FIG. 12: AX is theoptical axis of the projection optical system PL; and a rectangular areaLE, extending in the X direction, with optical axis AX as its center, isan illumination area of illuminating light during the scanning exposuretime.

In accordance with the preferred embodiments described above, reticle R,through the protrusion components of mounts 10B and 10D, which are in analmost diagonal position, is regulated in displacement along the Y axis,with the result being that the influence from the expansion of reticle Rappears in FIG. 12 as minute rotations in the clockwise direction ofreticle R. However, because in this preferred embodiment, corner mirrorsCM1 and CM2 are formed at the end of reticle R, the collectiverotational variations of reticle R, occurring from the expandedcomponent of reticle R, and from the shifting component on mounts10A-10D, can be accurately measured across a comparatively wide range.

Accordingly, when reticle R and wafer W are relatively moved in the Ydirection at a speed ratio equal to the image formation magnification ofthe optical projection system, so that exposure scanning is performed ofthe image of the pattern area of reticle R, illuminated by illuminationarea LE on the shot area of wafer W, adjustments can be performed of thesize (magnification) of the scanning direction of the entire image ofthe projected patterned area by finely adjusting the relative speedratio (between the reticle stage and wafer stage) by an amountcorresponding to the expansion amount of reticle R, with respect to theexact value of the image formation magnification.

The size adjustments concerning the non-scanned direction (X direction)of the entire image of the projected patterned area, as explained inFIG. 9 above, may be made by finely adjusting the image formationmagnification of the projection optical system PL itself, in response tothe expansion amount of reticle R. Moreover, the structure of thispreferred embodiment can be applied to an exposure apparatus formanufacturing a large-sized liquid crystal device, in which theprojection optical system has an image formation magnification of 1, andmask substrates and photosensitive substrates are maintained on a commoncarriage and are scanned one-dimensionally with respect to theprojection optical system. Such apparatus include, e.g., a multi-lensstyle of projection exposure device disclosed in Japanese Laid-OpenPatent Application No. 7-57986, the disclosure of which is incorporatedherein by reference in its entirety.

In this preferred embodiment, a temperature sensor for measuring theexpansion of reticle R, or interferometers for measuring reticledimensions, are not particularly used for performing measurement of theexpansion amount of reticle R; however, as necessary, a measuring systemmay be established to perform such measurement of the amount ofexpansion, either directly or indirectly, as in the previousembodiments.

The circuit pattern to be printed on the mask substrate and severalalignment marks are precisely positioned on the mask substrate usingside surfaces of the mask substrate as a reference. This facilitates thecontrol of position references in the projection exposure apparatus,which uses the mask substrate to transfer the pattern image onto thephotosensitive substrate. Because the mask stage supports the masksubstrate so as to allow for free expansion, the mask substrate isprevented from being warped even if the mask substrate absorbs a portionof the illumination energy and expands. This can further preventdistortion or warp of the image plane, which adversely affects thequality of the projected pattern image during projection exposure.Furthermore, measuring devices are provided to measure positionaldisplacement of the side surface of the mask substrate on the maskstage, and the mask substrate is moved based on the measuring results.Even if the mask substrate slightly slips out of the initial position onthe mask stage, the mask substrate can be precisely positioned to thedesired target position by correcting the displacement.

Expansion of the reticle R during the projection exposure is directlyand accurately measured, whereby alignment error, superposing error, andmagnification error in the projected image are corrected in real time.

While this invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, the preferred embodiments of the invention as set forthherein are intended to be illustrative, not limiting. Various changesmay be made without departing from the spirit and scope of the inventionas defined in the following claims.

What is claimed is:
 1. A mask comprising:a main surface and sidesurfaces located at a periphery of the main surface; a pattern formedsubstantially parallel to the main surface of the mask; and at least onereflective area provided at least on a portion of the side surfaces ofthe mask, the pattern being aligned with the at least one reflectivearea.
 2. The mask of claim 1, wherein the mask has a rectangular shape,and the at least one reflective area is formed on at least twoorthogonal side surfaces of the mask.
 3. The mask of claim 1, whereinthe mask includes a transparent plate having the main surface, and thepattern is formed of a light-blocking material layer deposited on themain surface.
 4. The mask of claim 1, wherein the mask includes atransparent plate having the main surface, and the pattern is formed ofa phase-shifter material layer deposited on the main surface.
 5. Themask of claim 1, wherein the at least one reflective area is a layer ofreflective material deposited on the side surfaces of the mask.
 6. Themask of claim 1, wherein a plurality of the reflective areas areprovided at distinct, spaced apart locations on the side surfaces of themask.
 7. The mask of claim 6, wherein at least one of the plurality ofreflective areas is partially light-transmissive.
 8. The mask of claim7, wherein a reflective area having a light-reflective surface thatfaces an internal portion of the mask is provided at a position on themask that is diametrically opposed to the at least one partiallylight-transmissive reflective area.
 9. The mask of claim 1, wherein theside surfaces of the mask include a notch, the at least one reflectivearea being provided in the notch.
 10. The mask of claim 1, wherein themask includes a right-angle corner, the at least one reflective areabeing provided on the side surfaces of the mask located at theright-angle corner.
 11. The mask of claim 10, wherein the at least onereflective area provided on the side surfaces of the mask located at theright-angle corner includes a reflective surface that faces toward aninternal portion of the mask.
 12. The mask of claim 1, wherein the maskincludes a cut-off corner, the at least one reflective area beingprovided on the side surfaces of the mask located at the cut-off corner.13. The mask of claim 12, wherein the at least one reflective areaprovided on the side surfaces of the mask located at the cut-off corneris partially light-transmissive.
 14. A projection exposure apparatuscomprising:an illumination optical system located between a light sourceand a mask illuminated with light from the light source; a mask stagethat mounts the mask to receive light from the light source; a substratestage that mounts a substrate having a plurality of areas that are toreceive light from the illuminated mask; a projection optical systemlocated between the mask stage and the substrate stage; and a maskmounted to the mask stage, the mask including a pattern formedsubstantially parallel to a main surface of the mask and at least onereflective area provided at least on a portion of side surfaces of themask, the side surfaces located at a periphery of the main surface, thepattern being aligned with the at least one reflective area.
 15. Aprojection exposure apparatus comprising:an illumination optical systemlocated between a light source and a mask illuminated with light fromthe light source, the mask having a pattern thereon; a projectionoptical system located between the mask and a substrate to project thepattern of the mask onto the substrate by the illumination from thelight source; a two-dimensional driving stage that mounts the substrateand moves the substrate with respect to a field of view of theprojection optical system; a mask stage having a plurality of mounts onwhich the mask is supported so that a main surface of the mask, alongwhich the pattern extends, is aligned with an object plane of theprojection optical system, the plurality of mounts contacting the maskat a plurality of positions along a periphery of the mask and allowingfor movement of the mask on the mounts along the main surface; a columnstructure that integrally supports the mask stage and the projectionoptical system; a measurement system that emits a measuring beam to areflective area formed on at least a portion of side surfaces of themask and that receives a beam reflected from the reflective area todetect information about positional changes of the reflective area ofthe mask; and a driving mechanism that slightly moves the mask relativeto the mask stage based on the information obtained by the measurementsystem.
 16. The apparatus of claim 15, wherein the measurement system isan interferometer system.
 17. The apparatus of claim 15, furthercomprising a temperature sensing system that detects temperatureinformation of the mask, and wherein the driving mechanism also movesthe mask stage relative to the column structure based on the temperatureinformation obtained by the temperature sensing system.
 18. Theapparatus of claim 15, wherein the measurement system emits two of thebeams and detects information about a positional change of reflectiveareas of the mask in two orthogonal directions.
 19. The apparatus ofclaim 15, further comprising a projection optical system controller thatcontrols a magnification amount by which the projection optical systemmagnifies the pattern of the mask projected onto the substrate.
 20. Aprojection exposure apparatus comprising:a two-dimensional driving stagethat mounts a substrate and moves the substrate; and a mask stage havinga plurality of mounts on which a mask is supported, a main surface ofthe mask having a pattern to be exposed onto the substrate, theplurality of mounts contacting the mask at a plurality of positionsalong a periphery of the mask, and allowing for expansion of the mask onthe mounts along the main surface without inducing stress in the mask.21. The apparatus of claim 20, wherein the mask stage includes a pushingmember that gently forces the mask supported on the plurality of mountsin a direction along the main surface, and the mask is maintained inposition on the plurality of mounts only by gravity.
 22. The apparatusof claim 20, further comprising at least one vacuum port that appliesvacuum to at least one of the plurality of mounts to hold the mask onthe mounts.
 23. The apparatus of claim 22, wherein at least some of theplurality of mounts do not have the vacuum applied thereto so that themask is free to slide thereon.
 24. The apparatus of claim 20, furthercomprising a driving mechanism that moves the mask relative to the maskstage.
 25. The apparatus of claim 20, further comprisinga measurementsystem that obtains information regarding changes in position of themask relative to the mask stage; and a driving mechanism that moves themask relative to the mask stage based on the information obtained by themeasurement system.
 26. The apparatus of claim 25, wherein themeasurement system emits a measuring beam to a reflective area formed onat least a portion of side surfaces of the mask and receives a beamreflected from the reflective area thereby detecting the informationabout changes in the position of the mask relative to the mask stage.27. The apparatus of claim 26, wherein the measurement system is aninterferometer system.
 28. The apparatus of claim 25, wherein themeasurement system includes a temperature sensing system that detectstemperature information of the mask, and wherein the driving mechanismmoves the mask relative to the mask stage based on the temperatureinformation obtained by the temperature sensing system.
 29. Apositioning apparatus for positioning a substrate which has a mainsurface and side surfaces located at a periphery of the main surface ofthe substrate, comprising:(a) a substrate bearing structure having atleast a portion of a drive actuator that moves the substrate in a planeparallel to the main surface of the substrate; (b) a measuring systemthat emits a measuring beam toward a reflective area provided on atleast a portion of the side surfaces of the substrate, that detects areflected beam from the reflective area of the substrate and thatgenerates a positional information representing a coordinate value or adisplacement amount of the substrate directly; and (c) a control systemcoupled between said measuring system and said drive actuator toenergize said drive actuator in response to said positional information.30. The positioning apparatus of claim 29, wherein said measuring systemis an interferometric measuring system.